Numerical modelling and experimental measurements for a low-temperature district heating substation for instantaneous preparation of DHW with respect to service pipes Marek Brand a, * , Jan Eric Thorsen b , Svend Svendsen a a Department of Civil Engineering, Technical University of Denmark, Brovej, Building 118, DK-2800 Kgs. Lyngby, Denmark b Danfoss District Energy, Nordborg, Denmark article info Article history: Received 1 October 2011 Received in revised form 23 February 2012 Accepted 26 February 2012 Available online 30 March 2012 Keywords: District heating Low temperature DHW Numerical model Simulation Bypass abstract Traditional district heating (DH) systems are becoming uneconomic as the number of new and renovated buildings with reduced heating requirements increases. To keep DH competitive in the future, heat losses in DH networks need to be reduced. One option is to reduce the supply temperature of DH as much as possible. This requires a review and improvement of a DH network, in-house substations, and the whole domestic hot water (DHW) supply system, with the focus on user comfort, hygiene, overall cost and energy efficiency. This paper describes some practical approaches to the implementation of low- temperature district heating (LTDH) with an entry-to-substation temperature around 50  C. To this end we developed a numerical model for an instantaneous LTDH substation that takes into consideration the effect of service pipes. The model has been verified and can be used for the further optimization of the whole concept as well for individual components. The results show that the way that the service pipe is operated has a significant effect on waiting time for DHW, heat loss, and overall cost. Furthermore, the service pipe should be kept warm by using a bypass in order to fulfil the comfort requirements for DHW instantaneously prepared. Ó 2012 Elsevier Ltd. All rights reserved. 1. Low-temperature district heating District heating (DH) is a way of providing buildings with heat for space heating (SH) and domestic hot water (DHW) prepara- tion in an economical and environmentally friendly way. Nowa- days, building regulations have been introduced worldwide and are helping to reduce energy consumption in buildings, because 40% of all energy consumption takes place in buildings. The energy policy of the European Union is currently focused on security of supply, energy savings, reducing production of CO 2 , and increasing the proportion of renewable energy [1]. DH is one of the best ways to achieve these goals in the building sector and its further development has high priority. According to Heat Plan Denmark 2008, DH systems supplied by renewable energy sources could cover up to 70% of heating demand in Denmark by 2050. The remaining 30% is assumed to be covered by individual heat pumps, installed mainly in areas with low heat demand density [2,3]. Nevertheless, traditional high and medium temperature DH systems are not optimal solutions for the future [4]. To accord with EPBD [5], all buildings constructed after 2020 must be near Zero-Energy Buildings [6] and this will create areas with lower heat demand than today. Table 1 shows the imple- mentation of EPBD for newly built residential buildings in Denmark valid since 2008 [7] and the update in 2010 [8]. The values of the maximum specific primary energy demand, account for space heating, DHW heating and electricity for operation of HVAC systems (pumps, fans). The DH networks in current use will not be able supply heat to these areas in an economical way, because the ratio between network heat losses and the heat consumption in the buildings would be unacceptably high, the cost of heat for the end-users would increase, and DH systems would cease to be competitive with other solutions, such as heat pumps [9]. Research in the field of DH has recently focused on the supply of areas with low heat demand and low-energy buildings [10] and on increasing the proportion of heat produced by renewable sources of energy, such as solar heat plants, geothermal energy, or heat pumps driven by electricity from renewable sources. Attention has also been paid to the smart grid concept, where buildings connected to DH network * Corresponding author. Tel.: þ45 45251884; fax: þ45 45883282. E-mail addresses: marek@byg.dtu.dk (M. Brand), jet@danfoss.com (J.E. Thorsen), ss@byg.dtu.dk (S. Svendsen). Contents lists available at SciVerse ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.02.061 Energy 41 (2012) 392e400